Efficient regulation of inflammation in the lungs is essential not only to allow for rapid mobilization of inflammatory cells during infection, but also to prevent inflammation during exposure of the lungs to innocuous substances. Normal homeostatic conditions in the lungs are generally thought to be immunosuppressive, due in part to alveolar macrophages (AM) which can release inhibitory factors including nitric oxide, prostaglandins, transforming growth factor and interleukin-10 (IL-10). Binding of IL-10 to IL-10 receptors on AM dramatically inhibits the production of proinflammatory cytokines, IL-1, IL-6, IL-8 and TNFalpha. However, we have shown that under inflammatory conditions, AM become hyporesponsive to IL-10 in that synthesis of TNFalpha and IL-6 cannot be effectively inhibited by IL-10. Bronchoalveolar epithelial cells (EpC) also have been shown to constitutively release IL-10, but to loose that capacity during inflammatory conditions. We hypothesize that under normal conditions in the lung, an inhibitory loop is active in which constitutively produced IL-10 by EpC acts on AM to prevent inappropriate synthesis of proinflammatory cytokines. Following exposure to infectious microorganisms, the homeostatic production of IL-10 by EpC and/or the response of the IL-10 receptors on AM are rapidly diminished via signaling through pattern recognition receptors on EpC and AM. Induction of proinflammatory cytokines in the lungs is thus more efficiently induced by microbial rather than by non-microbial substances. Using in vivo murine models and cell culture models, we will evaluate the following: 1.) What changes are induced in the synthesis of IL-10 by alveolar epithelial cells and in IL-10 receptor function on alveolar macrophages by microbial and non-microbial stimuli? 2.) Is induction of IL-10 hyporesponsiveness in alveolar macrophages mediated via pattern recognition receptors, including Toll-like receptors and phagocytic receptors? 3.) Is IL-10 receptor hyporesponsiveness mediated directly via inhibition of signal transduction pathways or indirectly via synthesis of inhibitory molecules such as "suppressors of cytokine signaling" (SOCS)? 4.) Can IL-10 hyporesponsiveness in vivo be ameliorated by inhibition of Toll-like receptor signaling?